1. Field of the Invention
This invention relates generally to dielectric resonator oscillators for generating microwave energy, and relates more particularly to a dielectric resonator oscillator utilizing transmission-type injection-locking for frequency stabilization.
2. Description of the Relevant Art
Injection-locking is a well known technique for stabilizing the oscillation frequency of an oscillator by locking it to the frequency of an injection-locking signal. The injection-locking signal is typically a harmonic of a low-frequency reference signal that has a stable frequency and high spectral purity, such as the output signal from a crystal oscillator. The injection-locking signal may be generated, for example, by first generating the reference signal, and then amplifying and multiplying the reference signal to generate its harmonics, and then filtering to remove all of the harmonics except for the harmonic at the desired frequency of the injection-locking signal. The injection-locking signal is then supplied to the oscillator for frequency stabilization.
In order for the injection-locking to work properly, the injection-locking bandwidth of the oscillator must be greater than the frequency drift of the oscillator over varying operating conditions of temperature, load, bias, and time. This typically involves a trade-off between the power of the injection-locking signal and the stability of the oscillator, with increasing power of the injection-locking signal needed to compensate for increasing drift of the oscillator.
Two types of injection-locking, reflection-locking and transmission locking, are distinguished by the point at which the injection-locking signal is injected into the oscillator. Reflection injection-locking involves the injection of the locking signal into the output port of the oscillator through a circulator. The circulator is needed in order to isolate the output signal of the oscillator and the injection-locking signal. Unfortunately, the use of the circulator introduces losses into the signal path of the oscillator and complicates the task of packaging because any output amplifier must be placed downstream from the circulator, instead of on the same substrate as the oscillator. Transmission injection-locking involves the injection of the locking signal into a port of the oscillator apart from the output port, thus requiring separate signal-input and power-output ports, but eliminating the need for a circulator.